Several possibilities here, the system requirements for your new card is:
Radeon X1950 pro
AGP based PC is required slot available on the motherboard
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Connection to the system power supply is required
450-Watt power supply or greater, 30 AMPS, 12 volt rail recommended
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512MB of system memory, or more recommended
CD-ROM drive (for installtion software)
DVD playback requires DVD drive and decoder software (not included)
Operating System:
- Windows XP
- XP-64
GeForce 5500
Windows 98SE/Me/2000/XP
Available AGP slot
CD-ROM drive (to load drivers CD)
As you can see, the old video card was designed to work with any power supply and did not require a direct connection from the power supply. Your new Radeon X1950 Pro requires a minimum power supply size of 450 watts AND 30AMPS plus a power connection directly off the power supply. Thus you need to check the rating on your current power supply to see if it conforms to the minimum total output of at least 450 watts and 30 amps. Also, did you install the card with the direct power feed from the power supply? It would be a plug like your hard drive uses that plugs directly into a slot on the new video card, so the card can draw power through the AGP port as well as directly from the power supply.
This is most probably your current issue, an insufficient sized power supply and/or having not supplied to direct power feed from the power supply. If you system is a pre-built from a manufacturer like Dell, Gateway, HP, etc, they only use the smallest power supply required to support the components they installed initially, if you add anything that draws power later, you almost always end up having to replace the power supply too.
Once you correct the power supply problem and the system starts up properly with video, you are going to need to goto Start, Control Panel, Add/Remove Programs and remove your nvidia drivers, once done will require you to reboot your system. You will know you did it right when you restart and all the icons are huge on the screen. Now goto: http://ati.amd.com/support/driver.html select the appropriate operating system in the first window, Radeon in the second, and then Radeon 1950 series in the last and press the green go button, this will allow you to download the most current video drivers for the new card. Once downloaded to your machine, run the program to install the current catalyst software for your new video card. Upon completion of this installation, the system should reboot yet again, and you should have everything working properly.